A tracking radar measures the coordinates of a target and supplies data which can be used for determining the path of the target and predicting its future position. To establish this prediction it is possible to use substantially all the data available in a radar, namely the distance, elevational angle, azimuth and Doppler frequency, so that a priori any radar can be looked upon as a tracking radar from the time when the output information which it supplies is processed in an adequate manner. However, a tracking radar differs from other radars by the way in which the angular tracking of the target takes place. This mode of tracking serves to generate an error signal indicating the angular deviation or squint of the target direction with respect to the axis of the antenna, referred to in the art as the boresight axis, this error signal being supplied to servomechanisms for realigning the antenna axis with the target direction.
In general, there are three standard methods of producing this error signal.
A first method is the detection of a target by sequential lobing, the second method is conical scanning, and the third method is the monopulse technique.
The antenna according to my invention uses the conical-scanning method, whose principle will now be described.
In a conical-scanning system having focusing means, the antenna is illuminated by a primary source and its phase center describes a circle of given radius located in the focal plane around the focal axis of the system. With such an antenna the radiation diagram is no longer centered on the axis of the focusing system but instead rotates in space in such a way that the maximum-radiation direction describes a cone whose vertex half angle is called the squint angle of the antenna. In the absence of focusing means, conical scanning can be obtained by a rotary source inclined with respect to its axis of revolution whose phase center is located on that axis.
The amplitude of the signal supplied by the antenna is thus modulated at the rotation frequency of the diagram and its modulation depth is a function of the angle between the target direction and the rotational axis. The modulation signal extracted from the echo signal is used in servomechanisms for locking the antenna position onto the target.
Owing to the symmetry of revolution, the beams radiated by the antenna all intersect along the axis of revolution and generally cross at a point chosen by way of compromise between the slope at the origin, determining the precision of pointing, and the range of the radar.
In a conventional conically scanning antenna the radiation diagram is the same on transmission and on reception; thus, by analyzing the diagram on transmission, it is possible to find the rotational frequency of the diagram for jamming purposes.
There are instances where this possibility of detecting the rotational frequency of the radiation diagram of the conically scanning antenna must be eliminated.
It has already been proposed to transmit in accordance with a radiation diagram centered on the axis of the antenna and to receive in accordance with a radiation diagram conforming to conical scanning. A construction based on this principle has a primary source of the monopulse type supplying signals in a sum channel and in two difference channels, one in azimuth and the other in elevation. The sum channel is combined with the difference channels and the conical-scan diagram is obtained on reception by a rotary variable phase shifter which varies the phase between the difference and sum signals. The radiation diagram obtained is eccentric and rotates at the velocity of the phase shifter. This arrangement defines a receiver with a single channel which is not, however, protected from errors with regard to the determination of the angles, due to fluctuations of the echo amplitude. Moreover, it involves relatively complex and consequently expensive structures.